1. Field of the Invention
The present invention relates to a method of producing aromatic amino azo compounds, e.g., p-aminoazobenzene, by the rearrangement of 1,3-diaryl triazenes, e.g., 1,3-diphenyltriazine.
2. Description of the Prior Art
Aromatic amino azo compounds are useful as solvent dyes and as intermediates in the preparation of aromatic diamines, which, in turn, are used in the preparation of polymers, antioxidants, etc.
U.S. Pat. No. 1,977,266 describes the reaction of sodium nitrite with solid aniline hydrochloride formed into a paste with water to form diazoaminobenzene by diazotization-coupling, followed by raising the temperature of the reaction mixture to slowly rearrange the diazoamino compound to 4-aminoazobenzene, the latter being obtained in the form of its hydrochloric acid salt. The rearrangement is carried out over a period of several hours at temperatures in the range of 20.degree. C. to 40.degree. C.
According to the process of U.S. Pat. No. 2,538,431, the rearrangement of the diazoamino to the amino azo compound is carried out in the same stage as the diazotization-coupling reaction in an alcohol medium. The diazotizing agent, i.e., a nitrite or nitrous acid, is admixed with a solution of the amine in hydrochloric acid and a restricted amount of an alcohol. A temperature of 10.degree. C. to 40.degree. C. and a reaction time of 15 to 45 minutes are critical features.
U.S. Pat. No. 2,894,942 describes the formation of aromatic amino azo compounds by adding an inorganic nitrite to a mixture of an excess of a primary aryl monoamine, a mineral acid, and a Friedel-Crafts catalyst while the temperature is 0.degree.-50.degree. C. (20.degree.-30.degree. C. preferred) whereby a portion of the amine is diazotized and the diazonium salt couples with unreacted amine to form the diazoamino aryl compound; and thereafter heating up to about 40.degree.-75.degree. C. (45.degree.-55.degree. C. preferred) for a period of about from one-half to three hours whereby the diazoamino aryl compound rearranges to the amino azo compound under the combined influence of the Friedel-Crafts catalyst and heat. Temperatures above 75.degree. C. are disclosed as accelerating side-reactions. Without the Friedel-Crafts catalyst, the rearrangement is disclosed as requiring over six hours to complete. The same process is described in Canadian Pat. No. 580,083, except that the mineral acid is used in excess, and the Friedel-Crafts catalyst is omitted.
British Pat. No. 1,430,366 also describes rearranging the diazoaminobenzene in a reaction mixture obtained by reacting aniline with nitrous acid generated by mixing an alkali metal nitrite with an aqueous mineral acid. For the rearrangement, the aniline solution of the diazoaminobenzene is treated with aniline hydrochloride at 20.degree. to 100.degree. C. for several hours.
In the continuous process described in U.S. Pat. No. 4,018,751, wherein reactant streams comprising (a) an aromatic primary amine, mineral acid, and water and (b) aqueous metal nitrite solution are passed through a tubular reactor, and a p-aminoazo compound is recovered from the exiting stream, the reactor temperature is 40.degree. to 100.degree. C. and the residence time therein less than ten minutes.
In all of the above-discussed prior art, the diazotization of the aromatic amine to produce the diazoamino compound needed for the rearrangement is carried out in the conventional manner with an inorganic nitrite and an acid, or with nitrous acid generated therefrom. Such processes, when considered in terms of large-scale operations, suffer from the disadvantage that the salt, e.g., sodium chloride, produced as a by-product of the diazotization reaction needs to be separated and disposed of. This disadvantage recently has been overcome with the discovery of a process for the diazotization-coupling of primary aromatic monoamines by means of a nitrogen oxide-containing gas mixture derived from the oxidation of ammonia. This new process is described in U.S. Pat. No. 4,020,052, issued Apr. 26, 1977, the disclosure of which is incorporated herein by reference. One desirable feature of the process is that it produces no salt by-products requiring troublesome salt separation procedures. Another is that, unlike the product obtained in the prior art processes, a triazene-containing reaction product in the form of a homogeneous liquid can be obtained, an advantage if the liquid is to be used per se for a subsequent triazene reaction.
When diazoaminobenzene, for example, is synthesized by the diazotization of aniline with a nitrogen oxide-containing gas, the reaction product obtained is comprised of 1,3-diphenyltriazene dissolved in aniline, this solution possibly also containing, depending on the composition of the nitrogen oxides used and other reaction conditions, small amounts of aminoazobenzenes, aminobiphenyls, other nitrogen-containing polynuclear compounds, water, and/or nitric acid. During the rearrangement of the 1,3-diphenyltriazene in this solution, side-reactions leading to the formation of polynuclear (chiefly binuclear) by-products such as aminobiphenyls may occur. Also, yield losses to the undesired o-aminoazobenzene isomer may result.
The prior art has not addressed itself to the problem of the formation of polynuclear by-products in the rearrangement of 1,3-diaryl triazenes in solutions obtained by treating primary aromatic amines with nitrogen oxide-containing gases, or to the question of optimizing the production of the p-aminoazobenzene isomer in this system. These problems must be recognized and solved, however, if the aminoazobenzene process is to achieve its potential. The minimizing of the polynuclear by-product content is important on two counts. First, polynuclear by-products per se represent a yield loss and generally are reflective of additional losses due to tar formation. Secondly, and most importantly, 4-aminobiphenyl, a possible binuclear by-product of the rearrangement reaction, has been recognized as having carcinogenic properties. Accordingly, it is important to suppress the formation of binuclear by-products as much as possible, a practical level for total binuclear by-products being below 2%.
"Binuclear by-products," as the term is used herein, consist predominantly of 2-, 3-, and 4-aminobiphenyls and diphenylamine, and trace amounts of other similar binuclear compounds having nitrogen-containing substituents.
A process is needed for rearranging a 1,3-diaryl triazene in a solution obtained by the diazotization-coupling of a primary aromatic monoamine by means of a nitrogen oxide-containing gas mixture, which process minimizes the formation of binuclear by-products and preferably also maximizes the para/ortho isomer ratio of the aromatic amino azo compound.